Brain Pathology Case of the Month - April 2002

DIAGNOSIS: PLEXIFORM NEUROFIBROMA OF THE FACIAL NERVE.

DISCUSSION:

Primary neurogenic tumors of cranial nerve VII are uncommon with the majority found intratemporally. Intracranial and intraparotid neoplastic involvement of cranial nerve VII is much less common. Of the benign facial nerve tumors, which include schwannomas and neurofibromas, schwannomas are the most common (13). The peripheral portion of the facial nerve within the parotid gland gives rise to five terminal branches (parotid plexus). The majority of the neurofibromas arising in this area, specifically the plexiform variant, have been described in children with the autosomal dominant disorder neurofibromatosis 1 (NF 1) or von Recklinghausen's disease. In the English-language literature, primary intraparotid facial nerve involvement by a plexiform neurofibroma is rare, representing only nine cases (2, 4, 5, 9, 10, 12). Of these, one case in an adult patient did not have stigmata of NF 1 or a family history (5). This case represents another solitary intraparotid plexiform neurofibroma of the facial nerve in an adult patient without a personal stigmata or family history of NF 1.

The clinical diagnosis of NF 1, established by the 1987 NIH Consensus Development Conference, is made by having any two of seven criteria, one of which is a plexiform neurofibroma (11). This lesion, which is considered pathognomonic of the disorder, typically involves deep-seated nerve trunks, and results in nodular proliferations of nerve fascicles. The proliferating cells, which include Schwann, fibroblast, and perineural-like cells are arranged haphazardly to expand nerve branches and eventually diffusely infiltrate the surrounding tissue. Besides the connection with NF 1, plexiform neurofibromas should be distinguished due to their risk of malignant transformation seen in up to 15% of patients. Depending upon their location, these lesions may cause significant mortality due to their compressive effect. Complete surgical excision should be performed; however, recurrences may occur.

Since these lesions are typically identified in early childhood, the majority of people with NF 1 are diagnosed before the age of 30, with only 8% diagnosed after the age of 26 (3). At least three cases of solitary plexiform neurofibromas, two in the skin and one in the parotid gland, have been reported in patients without other stigmata of NF 1 (1, 5, 7). Fisher et al suggests that they may not be pathognomonic of this disorder if solitary and involve the small nerve trunks of the dermis (7). However, one should also consider the segmental form of NF that is believed to result from a local somatic mutation (14). These patients have localized disease in which the cutaneous and neural changes are limited to one section of the body and a family history of neurofibromatosis is usually not present. In a review by Rawlings et al of cases of segmental neurofibromatosis, most of the cases consisted of multiple cutaneous nodules or involvement of deep large nerves (14). None of the cases reviewed involved a solitary plexiform neurofibroma of a cranial nerve as in our case.

In conclusion, neurofibromas and the even more rare solitary plexiform neurofibromas may arise in the facial nerve and present as a parotid mass. Most cases are in children with known neurofibromatosis; however, adults without other stigmata have also been reported.